\documentclass{report}

\usepackage{amsmath,amsthm}

\begin{document}

\begin{eqnarray}
CA_{i}\oplus  CX_{i} &=& \mod (PA_{i}^{*}+t\times u_{i})\oplus u_{i} \nonumber\\
&\oplus &  \mod (PA_{i}^{*},256)\oplus u_{i}
\end{eqnarray}

\end{document}